1. Field of the Invention
This invention relates generally to a passive thermal control coating for a spacecraft and, more particularly, to a thermal blanket for a spacecraft that includes an outer coating of silicon sputtered on a substrate.
2. Discussion of the Related Art
Spacecraft typically employ many sensitive and expensive components for performing various space functions, such as information transmission and optical imaging. When in space, the spacecraft is subjected to thermal radiation from the sun which heats the spacecraft and its components to a very high temperature which may damage the spacecraft. Visible light is the main source of thermal energy in the space environment. Therefore, it has heretofore been known in the art to provide thermal control coatings, commonly referred to as thermal blankets, over sensitive spacecraft components to reflect and emit harmful solar radiation to prevent these components from being excessively heated.
Thermal control of a spacecraft is typically provided by radiating heat from the external spacecraft surfaces. The solar heating and cooling of the spacecraft is determined by the quantity of energy absorbed and the quantity of energy emitted by the spacecraft. The bulk of the energy absorbed by the spacecraft when it is in space is from the peak of the solar radiation spectrum, which has a wavelength of approximately 400-600 nm. The bulk of the energy emitted is from the infrared spectrum, at wavelengths greater than 1000 nm. The applicable equation for solar heating is provided by: EQU aS.sub.1 E.sub.0=eS.sub.2 sT.sup.4, (1)
where a is the solar absorbance of the spacecraft surface, S.sub.1 is the solar absorbing area, E.sub.0 is the solar irradiance, s is the emittance of the spacecraft surface, S.sub.2 is the emittance area, s is the Stefan-Boltzman constant and T is temperature (absolute).
One known thermal blanket used for this purpose is a silvered Teflon laminate. Particularly, a plastic substrate made of Kapton, having a thickness of approximately 2 mils, provides strength and high emittance for use as a cold-biased single layer closeout. A polyester adhesive layer is deposited on the Kapton substrate to approximately 1.5 mils thick. Next, an Inconel layer is deposited on the adhesive layer to approximately 300 angstroms thick. A silver layer is then vacuum deposited on the Inconel layer to about 1350 angstroms thick. The outer layer is a five mil thick FEP type "A" Teflon. This configuration is effective in providing the desired solar absorbance and emittance to minimize solar heating and temperatures. However, thermal blankets of this type are made of expensive materials and are difficult to fabricate.
Another known thermal blanket for this purpose is a plastic sheet, such as Kapton, sputtered coated with a thin germanium film. The germanium film provides a desirable reflectance characteristic for reflecting visible light. The plastic sheet provides a desirable emissive characteristic for emitting infrared radiation that also acts to keep the heat from passing through the blanket. Generally, the germanium is sputtered onto the plastic sheet using known vacuum deposition processes. In an alternate version of this technique for thermal control, the plastic layer is painted onto the spacecraft surface, and then the germanium layer is vacuumed deposited on the painted plastic layer.
Prior to being attached to a spacecraft, large sheets of the germanium coated plastic are rolled up for storage. When a piece of thermal blanket is needed, it is cut to length and attached to the desirable spacecraft component. During the storage time, it has been found that germanium suffers from corrosive degradation when exposed to humidity. The corrosive degradation causes pitting of the germanium film and causes a haze to cover the film. This degradation effects the reflective and emissive characteristics of the sheet, often rendering it useless for its intended purpose. The thermal coating can be stored in an unrolled condition or in a controlled environment in an attempt to prevent such degradation, but these fixes also add cost.
What is needed is a thermal blanket for use in connection with a spacecraft or spacecraft component that does not suffer from corrosive degradation when in storage and is relatively inexpensive. It is therefore an object of the present invention to provide such a thermal blanket.